can you use an inverter if System Size exceeds it.

[mnlara]

I reviewed a Photovoltaic planset for permit application and I saw a discrepancy. System Size shown on the plan is 6.750 kw dc. 3 strings of (9) Solarworld SW-250 Mono solar module and they used an inverter PVI6000-OUTD-US-240. I ran my own calculation and it indicates possibility of output power limiting. I called up the designer and they said it is OK. Is this right? can you use an undersized inverter? If so why?

 

[jaggedben]

Yes, it is not a problem.

Safety wise it is not a problem because the inverter will limit the current throughput. Think of the inverter as the load on the PV array, in this case. There is no problem with having a power supply that exceeds the required load. You can double check inverter input specs to make sure, but this probably will always be the case at the power ratio you've mentioned.

Economics-wise, it is not necessarily a problem because a PV array will actually produce it's rated output only a very small amount of the time. It will only reach peak output in the middle of the day, and on most days peak will be less than 100% of its rating. Combine this with the fact that the inverter will be only about 96% efficient, then choosing an inverter with a rated output of between 85% and 95% involves losing very little potential energy production, probably no more than a couple percent, if any. So it will often be a better choice than choosing the next size up inverter whose output is greater than 100% of the array, because in that case you are paying for inverter capacity that will never be required.

In your case, your 6.75kW array can be expected to produce roughly $900 worth of electricity a year at average US rates. If the next size inverter is 7000w, and we assume $0.50 per watt for additional inverter capacity, that's $500 in additional inverter costs. If the inverter is expected to last 15 years, then you need to make $33.33 each year in additional production to cover that. That is 3.7% of expected production, which is probably more than you can actually expect to gain by upsizing the inverter.

The exact break-even point is a guessing game, but the design you were given is entirely reasonable.

 

[mnlara]

Thanks for the enlightment. This info is greatly appreciated especially for me a rookie Photovoltaic designer. It seems to point that the owner saved even more...versus getting the next larger inverter considering that inverters are very expensive.

 

[PWDickerson]

Theoretically, you can safely install a solar array of any size on a grid interactive inverter. The inverter will never output more than its rated continous power. When the array is capable of generating more power than the inverter can handle, "clipping" ocurrs (the current flattens out at the max rating). Typically designers will size the solar array at 100-120% of the inverter capacity. Some go even higher than this. I have analyzed a few installation I have done and determined that clipping begins to occur a few days per year when the array is sized at 110% of the inverter rating. I try to stay at or below this number.

 

[ggunn]

Theoretically, you can safely install a solar array of any size on a grid interactive inverter. The inverter will never output more than its rated continous power. When the array is capable of generating more power than the inverter can handle, "clipping" ocurrs (the current flattens out at the max rating). Typically designers will size the solar array at 100-120% of the inverter capacity. Some go even higher than this. I have analyzed a few installation I have done and determined that clipping begins to occur a few days per year when the array is sized at 110% of the inverter rating. I try to stay at or below this number.

Many things need to be considered in deciding how much to "overload" an inverter. One is the latitude at which the system is being installed; we generally overload more northerly systems with more PV. Another is the tilt angle of the array; if it is tilted significantly less than latitude, we overload the inverter more. Some clipping during peak production times may be acceptable if the added capacity the rest of the time more than makes up for it. There is no magic formula; it's a balancing act between system cost and kWh/kW.